Meditations on First Philosophy
By Rene Descartes
In Meditation One Descartes doubts the existence of external objects because he has come to realize that many of the things he believed to be true in his youth are in fact false opinions. He doubts the existence of objects because he has used his false opinions as foundations for everything else in life that he perceives to be true. He searches for certainty by doubting the foundations of his beliefs and by freeing his mind of all cares and prejudices. Descartes doubts the existence of external objects in the following manner: sense misperception, dream hypothesis and the evil genius.
Descartes states that he had perceived objects to be of truth because of his senses. External objects are available to one's senses and therefore they make us perceive things for what they are not. As a result our senses deceive us, and Descartes shows us through this meditation that we have no way of knowing external objects are real because our senses are not reliable. Therefore if you deny external objects and their existence on the grounds of misperception of your senses, then in turn you are denying your own body. Because our senses are illusory we then have no way of knowing whether or not we are awake or hallucinating. If we can dream anything we can perceive, then you have no way of knowing that you are not dreaming at any given moment. Therefore you can assume you are always dreaming, in which case the objects you perceive to be in space and external are actually objects in your mental state of consciousness. Consequently those objects perceived as external are in fact a state in your mind and become internal objects.
Descartes states in this first meditation ...